Epidemiologic studies of recent dengue outbreaks on Pacific islands have shown that severe and fatal disease can result from a first infection with a dengue virus. These observations are not in accord with the widely held belief that such disease results only from a second or third dengue infection and is the result of some sort of "immunologic enhancement". The epidemiologic studies also have shown that both dengue types 1 and 2 can be propagated in nature by Aedes polynesiensis in the absence of the classical vector Aedes aegypti. In laboratory studies it was shown that Toxorhynchites amboinensis, a very large and hardy species of mosquito which does not feed on blood, was an excellent laboratory host for the detection and propagation of dengue and several other arthropod-borne viruses.